Caching via system.web.httpruntime.cache with file dependency is not refreshing

See more:

C#

cache

dependency

VS2015

Hello everybody

i have problems with some code :
I try to cache some value from a config file in the global.asax, with a timer.
The global goal of the project is to check some data and push messages for all webusers (via SignalR) every 20s.

i add dependency on a file, but when updating this file, the cache is not refreshing. i add a absoluteExpiration which is working well.

in the Application_Start of global.asax, i init an application timer:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();

    WebApiConfig.Register(GlobalConfiguration.Configuration);
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    BundleConfig.RegisterBundles(BundleTable.Bundles);

    RouteTable.Routes.MapHubs();
    RouteConfig.RegisterRoutes(RouteTable.Routes);

    var aTimer = new System.Timers.Timer(1000);
    aTimer.Elapsed += aTimer_Elapsed;
    aTimer.Interval = 20000;
    aTimer.Enabled = true;

}

the timer tick every 20s (working) :

void aTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{

    //occur++;

    double currNPM = getCurrentnParMinute(999);


    //UserActivityHub.TimerisTicked(occur);

}

my main function : return a double found in the config file, or in the cache (which is refreshing every 1 minute (ok) or when the file is updated (not ok))

  public double getCurrentnParMinute(int idstructure)
        {
            try
            {
                string cacheName = "getCurrentnParMinute." + idstructure;
                if (System.Web.HttpRuntime.Cache[cacheName] == null)
                {
                    string pathConfig = "c:\myconfig.xml";
                    XmlDocument xmlconfig = new XmlDocument();
                    xmlconfig.Load(pathConfig);

                    double toreturn = 123; /* not real code here, in real i extract this value from pathConfig.file (working) */


                    //CacheDependency dep = 
                    //System.Web.HttpContext.Current.Cache.Insert(cacheName, toreturn, dep); /* not working in global.asax */
                    System.Web.HttpRuntime.Cache.Insert(cacheName, toreturn, 
new CacheDependency(pathConfig), /* that's NOT ok*/
DateTime.Now.AddMinutes(1), /* that's ok*/
System.Web.Caching.Cache.NoSlidingExpiration);
                    return toreturn;
                }
                else
                    return (double)System.Web.HttpRuntime.Cache[cacheName];



            }
            catch (Exception ex)
            {
                return 0;
            }
        }

i have used this system often with no problem on differents web site, the difference here is that i need to use System.Web.HttpRuntime.Cache instead of System.Web.HttpContext.Current.Cache, because (if i understand well) System.Web.HttpContext.Current is null in the global.asax... Any idea what i'm doing wrong ?

What I have tried:

tried to update the file to refresh the cache, no success